1. The Field of the Invention
The present invention relates to guide wires, particularly to guide wires used to guide a catheter in a body lumen such as a blood vessel.
2. The Relevant Technology
Guide wires are used to guide a catheter for treatment of intravascular sites, such as percutaneous transluminal coronary angioplasty (“PTCA”), or in examination such as cardio-angiography. A guide wire used in the PTCA is inserted into the vicinity of a target angiostenosis portion together with a balloon catheter, and is operated to guide the distal end portion of the balloon catheter to the target angiostenosis portion.
A guide wire needs appropriate flexibility, pushability and torque transmission performance for transmitting an operational force from the proximal end portion to the distal end, and kink resistance (resistance against sharp bending). To meet such requirements, superelastic materials such as a Ni—Ti alloy and high strength materials have been used for forming a core member (i.e., a wire body) of a guide wire.
Near equiatomic binary nickel-titanium alloys are known to exhibit “pseudoelastic” behavior when given certain cold working processes or cold working and heat treatment processes following hot working Pseudoelasticity can be further divided into two subcategories: “linear” pseudoelasticity and “non-linear” pseudoelasticity. “Non-linear” pseudoelasticity is sometimes used by those in the industry synonymously with “superelasticity.”
Linear pseudoelasticity typically results from cold working. Non-linear pseudoelasticity results from cold working and subsequent heat treatment. Non-linear pseudoelasticity, in its idealized state, exhibits a relatively flat loading plateau in which a large amount of recoverable strain is possible with very little increase in stress. This flat plateau can be seen in the stress-strain hysteresis curve of the alloy. Linear pseudoelasticity exhibits no such flat plateau. Non-linear pseudoelasticity is known to occur due to a reversible phase transformation from austenite to martensite, the latter more precisely called stress-induced martensite (“SIM”). Linear pseudoelastic materials exhibit no such phase transformation. Linear pseudoelastic nickel titanium alloy can be permanently deformed or shaped by overstressing the alloy above a plateau stress that is at least partially dependent on the amount of cold-worked martensite structure present in the linear pseudoelastic structure. This is in marked contrast to non-linear pseudoelastic nickel titanium alloy, which cannot be permanently deformed by overstressing.